It is common practice in a computer system to have a number of I/O peripheral devices connected through one or more channels to computer systems. This is true with respect to main frame, mid-size and micro-computers. Typically, there are more I/O devices used with a computer than channel interfaces, so in the past it has been necessary to provide an external switch to selectively connect individual I/O devices to a channel as the need arises, or a "daisy chain" of the devices. Also, there was a trend toward having multiple channels to facilitate the various transfers of data back and forth between the computer system and the I/O devices such as printers, terminals, external storage, and the like. Of course, adding additional channels and/or providing an external switch can be expensive. Moreover, when an error occurred anywhere in the system, it was reported as immediate and damaging without regard to where the error had arisen. As a result, overall machine availability was adversely affected even though the area where the error arose was not in use at the time. Thus there is a need to accommodate a growing number of I/O devices without increasing the manufacturing and maintenance expenses of the system, and without proliferating the number of channels required. And when an error occurs, it would be desirable to isolate the error immediately and report it to the rest of the system only to the extent the rest of the system is affected.